Why does the lack of a circulatory system limit the size of free-living flatworms?

The lack of a circulatory system limits the size of free-living flatworms because they rely on diffusion to supply oxygen and nutrients to their cells and to remove metabolic waste.123

Because diffusion is only efficient over very short distances—typically less than 0.5 mm—organisms without a specialized vascular system are physically constrained by the need to keep their tissues close to the external environment. To overcome these limitations without a circulatory system, flatworms have evolved specific adaptations to reduce diffusion distances, including:3

• Dorsoventral flattening: Their thin, ribbon-like body shape ensures that no cell is too far from the surface.2
• Ramifying gut ceca: A highly branched digestive cavity helps distribute nutrients throughout the body.2
• Metabolic adjustments: Some species utilize glycolytic pathways that allow them to function in low-oxygen environments, reducing their immediate reliance on oxygen transport.2

These features allow flatworms to maintain their metabolic rates despite the absence of an independent circulatory system, but they remain restricted in size because any significant increase in thickness would prevent essential gases and nutrients from reaching internal cells.23